The present invention relates to controls for hydraulic actuators and more particularly to hydraulic valves and controls for such valves.
Hydraulic actuators are used in many mobile applications such as aerial platforms, earth moving equipment, cranes, and the like. The actuators are work cylinders or motors that are controlled by suitable hydraulic controllers. In some instances, the controllers are positioned for ease of plumbing and actuated from a remote location such as an operator cab. The remote actuation may be accomplished hydraulically or electrically.
In typical remote controlled valve systems a plurality of three- or four-way, hydraulic directional control valves are grouped in a valve bank. The bank is defined by a valve body having a plurality of bores for receiving individual valve spools. The valve body defines an inlet pressure port and a tank port. The inlet port is connected to a source of hydraulic fluid under pressure. In four-way, open center valve systems, each valve includes a pair of cylinder ports or outlets and a pair of tank or drain outlets. When a valve spool is in a neutral position, fluid flows through the valve directly from the inlet port to the tank port. A load holding check valve is disposed between an inlet port passage and an outlet pressure passage which is connectable to the cylinder ports by the valve spool. As the valve spool is displaced in response to an input signal, flow is restricted raising input pressure and causing the load control check valve to move to an open position. Fluid will flow from the inlet passage to the outlet pressure passage and around the spool to one of the cylinder ports. The valve spool opens one of the cylinder ports to input fluid flow while opening the other cylinder port to a tank, drain or return passage.
In many applications, electric or hydraulic remote control is provided for the hydraulic actuator control valves. In many such applications, a flow rate from the control valve which is approximately proportional to an input signal such as a control handle position would be desirable. Flow rate through a control valve at a given spool position will, also, vary with changes in input and/or output pressures. The flow rate through an hydraulic control valve is generally proportional to the square root of the differential pressure across the valve spool. Pressure changes can occur as the result of loads on the working cylinder and due to opening of additional valves in a valve bank. In order to maintain a predetermined or fixed flow rate for a given input or control signal, the valve spool must be shifted to compensate for such variances in pressure. Lack of compensation results in erratic or jerky actuator and hence equipment operation.
Individually pressure compensated control valves have been proposed. An example of one such valve may be found in U.S. Pat. No. 4,049,232 entitled PRESSURE COMPENSATING FLUID CONTROL VALVE and issued on Sept. 2, 1977 to the present inventor. The control valve disclosed therein includes a valve body having a valve spool mounted for movement from a neutral or open center position wherein fluid flows through the valve body to an operating position wherein the flow of fluid through the valve is restricted to direct inlet fluid through a flow control orifice to a pressure passage within the valve body. The pressure passage is connected to one of a pair of cylinder passages or ports. The other cylinder passage is connected to a tank passage. Pressure compensation to maintain a predetermined flow rate through the control orifice is achieved by imposing inlet and pressure passage pressure selectively to each end of the valve spool so that the spool responds to the pressure differential across the control orifice. The flow rate between the inlet passage network and the pressure passage is controlled by either a low force proportional solenoid valve assembly or a mechanically actuated assembly Another example of an hydraulic system may be found in U.S. Pat. No. 4,031,813 entitled HYDRAULIC ACTUATOR CONTROLS and issued on June 28, 1977 to Walters et al. A device for controlling an hydraulic actuator is disclosed which includes two servo loops having a fluid pressure operated main valve and an electrically operated pilot valve for controlling the main valve. One of the servo loops provides flow control and the other servo loop provides force control.
Presently available remote controlled directional valve systems suffer from various problems. Most systems have unpredictable metering ranges. Predictability can be achieved with some systems only by closely maintaining tolerances on the housing, metering lands, valve spools, springs and other components. In addition, hysteresis due to friction on the valve spools will cause a wide dead band in the remote control where movement of an input lever does not change flow rate. The available metering range will also decrease as system pressure increases. Present individually pressure compensated systems are also complex, difficult to manufacture and assemble and, hence, relatively costly. A need exists, therefore, for hydraulic or electric remote controlled valves for hydraulic actuators whereby the aforementioned problems are overcome.